1. Field of the Invention
The present invention relates to a backlight unit and, more particularly, to a direct type backlight unit suitable for use in a thin-model display device of a liquid crystal and the like.
2. Description of the Related Art
Liquid crystal display devices are widely used as monitors of office automation equipment and television sets and the like in terms of their characteristics, such as small size, thin design and low power consumption. The liquid crystal display device is constituted by a liquid crystal panel, in which a liquid crystal is supported by being sandwiched between opposed transparent substrates, a backlight unit which generates backlight illuminating the liquid crystal panel, and the like. As the kinds of backlight units, there have been known the direct type, the edge light type, the surface light source type and the like. The direct type is such that a light source is arranged on a rear surface of a liquid crystal panel and the light emitted from the light source is reflected on a reflector. The edge light type is called also the side light type. In this type, a light source is arranged on a side surface of a liquid crystal panel and the light emitted from the light source is guided on the whole rear surface of the liquid crystal panel by use of a light guide plate. The surface light type is such that a surface light source is arranged on a rear surface of a liquid crystal panel and the whole rear surface is illuminated. Usually, the direct type is used in large-sized liquid display devices.
For example, as shown in FIG. 1, a conventional direct type backlight unit 100 is constituted by a plurality of a lamp 5, a lamp support 6, a lamp connector 7, a reflector 8, a diffuser 4, an optical sheet 3, a center frame 2 and a rear frame 10. In FIG. 1, a liquid crystal panel (not shown) is assembled on the upper side of the backlight unit 100, whereby a liquid display device is constituted.
The lamp 5 is constituted by a plurality of cold cathode fluorescent lamps and the like. A pair of, lamp supports 6 supports end portions of the lamp 5. The lamp connector 7 is a connector for connecting the lamp 5 to an external circuit such as an inverter lighting circuit. The reflector 8 causes the light emitted from the lamp 5 to be reflected in the direction of the liquid crystal panel surface. The diffuser 4 is provided in order to diffuse the direct light from the lamp 5 and the reflected light from the reflector 8, whereby uniform light is made. The optical sheet 3 is constituted by a lens sheet, a polarizing sheet and the like. The center frame 2 and the rear frame 10 constitute a casing or a cabinet which holds the above-described components.
FIG. 2A is a plan view which shows the positional relationship of main members, including the lamp 5 and the reflector 8, in the conventional structure of FIG. 1. FIG. 2B is a sectional view which schematically shows the section along the line IIB-IIB in FIG. 2A as viewed from the direction of the arrow. As shown in these figures, the reflector 8 formed from a material having uniform reflectance is arranged on an inner bottom surface and inner inclined surfaces of the rear frame 10, whereby a lamp house is constructed. In the inner space in the interior of this lamp house, there are arranged plural linear lamps 5 at substantially fixed intervals substantially in parallel to each other.
The cold cathode fluorescent lamp used in this lamp 5 converts ultraviolet rays generated by discharges across electrodes provided at both ends into visible rays by use of a fluorescent substance applied to an inner wall of a glass tube. In this cold cathode fluorescent lamp, in general, the discharge condition across the electrodes is not uniform in the longitudinal direction, and regions of weak discharge are present near the electrodes. For this reason, usually, the luminance is low in an electrode near region A (FIG. 2A), which is in the vicinity of the electrodes of the lamp 5, and the luminance is high in a middle region B (FIG. 2A) in the longitudinal direction, which is positioned in the middle in the longitudinal direction of the lamp 5. As a result, even when the diffuser 4 is arranged, it is impossible to uniformly illuminate the liquid crystal panel, there by posing the problem that display quality decreases.
To solve this problem of a decrease in luminance in the electrode near region A, there has been proposed, for example, a backlight unit disclosed in Japanese Patent Laid-Open publication No. 1999-84377A. In this backlight unit, in a frame provided with a roughly rectangular bottom surface and side walls surrounding the bottom surface, which together form a lamp house, all of the side walls are provided with an inclination. That is, the inner surfaces of two side walls parallel to a linear light source (a lamp) are inclined surfaces in a direction in which the inclined surfaces open from the bottom of the frame toward the liquid crystal panel, and the inner surfaces of two side walls perpendicular to the linear light source are inclined surfaces in a direction in which the inclined surfaces close, inversely, from the bottom of the frame toward the liquid crystal panel.
In a backlight unit disclosed in Japanese Utility Model Laid-Open publication No. 1988-8777A, a reflecting section is provided with an uneven reflecting surface. That is, in the longitudinal direction of a light-emitting section (a lamp), both end portions where light-emitting luminance decreases have a concave shape and the middle portion where light-emitting luminance increases has a convex shape. On the other hand, in the side-surface direction (lateral direction) of the light-emitting section, the backlight unit has a roughly concaved shape which includes the circumference of, at least, the lower part of this light-emitting section and has an inclined surface which directs reflected rays toward the display panel.
By using these conventional constructions, it is possible to suppress a decrease in luminance in the region near the electrode of the lamp. In the former construction, however, the two inner surfaces of side walls perpendicular to the lamp provide inclined surfaces in the direction in which the inclined surfaces close from the bottom of the frame toward the liquid crystal panel and, therefore, in the longitudinal direction of the lamp the backlight unit protrudes outward from the liquid crystal panel. As a result, this poses the problem that the size of a frame outside the display area of the liquid crystal panel increases.
In the latter construction, because the reflecting section is worked in wave form, the construction becomes complicated and the manufacturing cost increases. Furthermore, the uneven construction of the reflecting section poses the problem that the thickness of the backlight unit increases inevitably.
In addition to the problem of a decrease in luminance in the longitudinal direction of the lamp 5, there is another problem that luminance decreases in the arrangement direction of the lamp 5 arranged in multiple numbers. That is, in a middle region D (FIG. 2A) in the lamp arrangement direction, which is positioned in the middle of the arrangement direction of multiple lamps 5, luminance is high because illumination is performed by at least two adjacent lamps 5. In contrast to this, in a region C (FIG. 2A) near the end lamp, which is in the vicinity of the lamp 5 arranged in the end portion of the arrangement direction, illumination is performed by only one lamp 5 in the end portion and, therefore, luminance is low compared to the middle region D. For this reason, usually, such a decrease in luminance is suppressed by increasing the reflectance of the side wall (an inclined surface) of the frame outside the end lamp. However, in a case where the spacing between the end lamp and the inclined surface is large, a decrease in luminance cannot be sufficiently suppressed by the reflection from the inclined surface alone.